build: moving rest config subscription to extensions (#26469)

Risk Level: low
Testing: n/a
Docs Changes: n/a
Release Notes: inline

#26450
Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ d32189df676e84be7d0328dcaf6df889e11ceb6e

envoy/config/core/v3/config_source.proto

@@ -281,3 +281,7 @@ message ExtensionConfigSource {
   // if they do not match any type URL in the set.
   repeated string type_urls = 4 [(validate.rules).repeated = {min_items: 1}];
 }
+
+// A placeholder proto for the registration for the REST subscription implementation.
+message RestSubscription {
+}